The present invention relates to an optical head which is simple in construction, minimizes optical components, employs only one beam splitting surface and is useable with a magneto-optic recorder in a ruggedized environment with the capability of monitoring power level presented to the recording medium and monitoring focussing separately from the digital data output.
Optical recording heads are well known in the art as are magneto-optic recorders. In such systems, light, from a source such as a laser diode, is often directed through a beam splitter to a recorder which may take the form of a magneto-optical recorder comprising a substrate with a magnetic film formed thereon. The light from the laser source is polarized and when focused on the magnetic film in the vicinity of a switchable magnetic field it produces a heating effect which causes the film to assume a direction of magnetization in a first or second sense depending on the magnetic field. Subsequent reflection of polarized light from this surface rotates the polarization slightly in a first or second sense. The reflected light is directed therefrom to a suitable detector which can determine the polarization sense and use this information for providing a digital data output. Descriptions of such basic systems may be found in a book entitled "Optical Recording A Technical Overview" by Allen Merchant and published by Addison-Wesley Publishing Company. Section 4.1 (pp. 68-74) describes magneto-optical systems and Section 8.4 (pp. 209-219) describes concepts for magneto-optic heads. Another example may be found in the Tomita U.S. Pat. Nos. 4,639,816 issued Jan. 27, 1987 and 4,569,035 issued Feb. 4, 1986. In these basic systems, although some monitor power level and focus, no provision are made for monitoring the focus of the objective lens so as to maintain the proper focus of the system on the magnetic film separately from the digital data output and no provisions are made for monitoring the power level of the input light so as to maintain this level relatively constant at the magnetic film during operation. A system which monitors the power and controls the output of the laser input in accordance therewith, and which monitors the focus condition of the objective lens may be seen in the Wilson et. al U.S. Pat. No. 4,799,210 issued Jan. 17, 1989. This system, however, does not use polarization rotation for it data recording. Furthermore, it monitors the power level before the beam is split so that the power level is not truly representative of the power reading at the recording medium. Also it does not separately monitor the focus of the objective lens with the result that "cross talk" is introduced in the output signals. Furthermore, the system requires there be at least two beam splitting surfaces for the light paths involved and because considerable difficulty and cost is associated with creating a proper polarized beam splitting (PBS) surface such systems are overly costly. It should also be noticed that the number of optical parts involved in the prior art systems are relatively high and it is desirable to reduce the number of optical parts to a minimum as is explained in an article by Y. Yamanaka et al. entitled "Compact Magneto-Optical Disk Head Integrated With Chip Elements" from the SPIE Vol. 1499 entitled "Optical Data Storage '91" by James J. Burk et al. sponsored by the Optical Society of America.